Field of the Invention
The present invention relates to a multilayer composite comprising a barrier layer of polyester and a layer of a moulding compound based on an impact-modified partly aromatic polyamide. The multilayer composite is primarily a hollow article, for instance a hollow profile or a container for conducting or storing liquid or gaseous media.
Discussion of the Background
The development of multilayer composites employed, for example, as pipes for conducting liquid or gaseous media in motor vehicles is subject to automotive industry demands for an improved barrier effect, for fuel lines in particular, to reduce emissions of fuel components into the environment as well as to requirements of sufficient fuel resistance. This has resulted in the development of multilayer pipe systems where, for example, thermoplastic polyester is employed as the barrier layer material. Such multilayer composites comprising not only a polyester layer but also further layers based on aliphatic polyamides are known from U.S. Pat. No. 6,355,358 for example.
Since there is a trend for higher temperatures in automotive engine bay applications the heat distortion temperature of aliphatic polyamides is often no longer sufficient for such applications. Efforts to replace aliphatic polyamides with partly aromatic polyamides have therefore been ongoing for some time. For instance, WO 2005/018891 describes hollow articles comprising at least one layer of an impact-modified partly aromatic polyamide; said articles may further comprise layers of an aliphatic polyamide but no further layers. This document teaches that, compared to PA 12, impact-modified partly aromatic polyamides have an improved barrier effect toward the diffusion of fuel components. However, for many applications, particularly at relatively high temperatures, this barrier effect is not yet sufficient.
JP 2011 230503 A discloses that fuel lines may comprise a layer of a partly aromatic polyamide and a barrier layer made of polyalkylene naphthalate. US 2003/0192612 A1 moreover describes pipes comprising an outer layer of aliphatic polyamide, an interlayer of polyphthalamide and an inner layer of polyester. The type of polyphthalamide employed is not disclosed.
Polyamide-polyester composites where the polyamide has a crystalline melting point Tm of at least 220° C. and sufficient crystallinity would be desirable for automotive engine bay applications. These criteria are met by commercially available, crystalline partly aromatic polyamides. However, said polyamides are unsuitable for such applications because of their poor mechanical properties, in particular their poor impact resistance and low elongation at break. EP 2857456 A1 discloses measurements on moulding compounds composed of a PA6T/6I/66 and of a PA10T/TMDT, each comprising 30 wt. % of different impact modifiers; the elongation at break is 3% to 6%. US 2014/0299220 A1 sheds further light; comparative example 22 therein shows a pipe comprising an 800 μm-thick layer of an impact-modified PA6T/6I/66 and a 200 μm-thick layer of an ETFE, the elongation at break of the pipe being 13%. Comparative example 24 shows a corresponding pipe where the polyamide layer is composed of an impact-modified PA9T whose diamine fraction is a 50:50 isomer mixture of 1,9-nonanediamine and 2-methyl-1,8-octanediamine; the elongation at break here is 22%. Finally, comparative example 27 shows a corresponding pipe where the polyamide layer is composed of an impact-modified further PA6T/6I/66; the elongation at break here is 18%. However, an elongation at break of the pipe of more than 100% is desirable.